1. Field of the Invention
This invention relates to a printing apparatus for printing pictures and characters used in printers, facsimiles and the like.
2. Description of the Prior Art
As one of the printing methods used to obtain printed matter directly from electric signal information representing characters or pictures, a method of combining electrosensitive recording and thermal copying has been proposed as disclosed in Japanese Unexamined Patent Publication No. 54-17036. Hereunder an example of this conventional printing apparatus will be described by referring to FIG. 3.
FIG. 3 shows a schematic construction of the conventional printing apparatus, in which an ink sheet 1 is composed of a transparent support 2, a conductive light reflective layer 3 (for example, an aluminum deposition layer ) provided on one surface of the support 2 and a thermoplastic ink layer 4 (for example, wax containing carbon) provided on the other surface of the support 2. The apparatus comprises a recording electrode 6, a grounded return circuit electrode 7, a recording signal source 8, a light source 9 for transfer, a transparent glass plate 15, a pressure pad 11 for intermittently pressing the ink sheet 1 and a recording paper 5 together, a spring 13 for pressing the pressure pad 11, and rollers 12 for transferring the ink sheet 1.
First, in the electrosensitive recording process, a recording voltage is applied to the ink sheet 1 via the recording electrode 6 so that the conductive light reflective layer 3 is partly removed by a discharge impulse current. The, the ink sheet 1 is overlaid with the recording paper 5 to begin the transfer process. During the transfer process, the pressure pad 11 is actuated to press the recording paper 5 and ink sheet 1 tightly onto the glass plate 5 to keep them in contact with each other, and the light source 9 emits light to melt the ink layer 4 at the part corresponding to the removed part of the conductive light reflective layer 3, whereby the molten ink is transferred to the recording paper 5. The pressure pad 11 is released after each transferring process so that the overlaid ink sheet and recording paper can be moved for the next transferring process.
In this printing apparatus, however, in order to melt the ink layer 4 at the part exactly corresponding to the removed part of the conductive light reflective layer 3 by the discharge impulse current and transfer the molten ink to the recording paper 5, it is necessary to keep the ink sheet 1 and recording paper 5 in tight contact with each other. If this contact is insufficient, uneven transfer occurs, which may impair the printing quality. Accordingly, a large load (at least 400 g/cm.sup.2) is required to press the pressure pad 11 onto the glass plate 15, which results in an increase of the size of the apparatus. Furthermore, since it is necessary to apply the pressure intermittently, the movement control mechanism is complicated.